1. Field of the Invention
The present invention refers to a device and a method for encrypting and decrypting a block of data known as a block cipher, the size of the input block and output block being the same.
This operation is controlled using a key which could either have the same size as the block or could have a different size, generally a larger size.
This invention refers to a symmetrical encryption/decryption method as opposed to the asymmetrical method. The symmetrical method is characterized by using the same key to encrypt and decrypt the data while the asymmetrical method uses a first key to encrypt and a second key to decrypt the data.
2. Description of the Related Art
Well known methods include DES (56 bit key), CAST (128-bit key), Blowfish (448-bit key), Twofish (256-bit key), and Rijndael (also known as AES, 256-bit key). Depending on the applications concerned, they have their own advantages and disadvantages.
Several patents have been published describing these methods. U.S. Pat. No. 5,214,703 describes the method known as IDEA™ which is based on a 8.5 rounds operations encryption process for 64 bits block length, each round using 6 sub-keys derived from the main key. The core is constituted by a Lai-Massey scheme using addition modulo 216, multiplication modulo 216+1 and bitwise excusive-OR.
The two major requirements for an encryption method is the robustness against any form of cryptanalysis and the computational speed. One key factor for the robustness is achieved by the diffusion effect, i.e. when one bit is changed in the input data, all the output bits are influenced in an unpredicted manner.
The computational speed is mainly determined by the type of mathematical and logical operations needed. More complex operations (division, multiplication) may prolong the time to execute the encryption process.